1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer and a manufacturing method of the image forming apparatus, and more specifically an image forming apparatus which has a plurality of recording heads and a manufacturing method of the image forming apparatus.
2. Related Background Art
An image forming apparatus such as an LED printer forms a latent image by imaging a light beam emitted from an LED array onto a surface of a photosensitive drum by way of a rod lens array.
FIG. 2 is a schematic diagram showing a photosensitive drum and surroundings of an image forming apparatus using an LED array as an exposing light source.
In FIG. 2, the LED array is disposed in a direction perpendicular to a rotating direction of the photosensitive drum, LED elements of the LED array emit rays in correspondence to image information, rays from the LED elements are imaged onto a surface of the photosensitive drum by way of an imaging optical system disposed before the LED array and an electrostatic latent image is formed two-dimensionally on the surface of the photosensitive drum by rotating the photosensitive drum.
A rod lens array is generally used as an imaging optical system in an image forming apparatus which uses such an LED array and disposed in the image forming apparatus is an LED head which is composed of the LED array integrated with the rod lens array consisting of rod lenses regularly arranged linearly in one or two rows between two plates.
FIG. 3 is a diagram descriptive of the rod lens. When a working distance which is a distance as measured from an end surface of the rod lens to an object or an image surface is denoted by L0 and a length of the rod lens itself is denoted by Z0, a conjugate length Tc of the rod lens is:
Tc=Z0+2 xc3x97L0. 
An image transmitting characteristic of this rod lens is evaluated as a quality of transmitted image, that is, resolution. This characteristic is expressed by a modulation transfer function (MTF). The MTF is an index which indicates a reproduction fidelity degree to an original image of an image which is formed after a rectangular pattern image shown in FIG. 4, for example, passes through the rod lens array. From FIG. 4, the MTF is defined as follows:
MTF(w)=(i(w)maxxe2x88x92i(w)min)/(i(w)max+i(w)min)xc3x97100% 
wherein i(w)max and i(w)min respectively denote a maximal value and a minimal value of a rectangular wave response at a spatial frequency of w(lp/mm). The MTF which is closer to 100% indicates image reproduction with higher fidelity to an original image. In addition, xe2x80x9clpxe2x80x9d in a unit of the spatial frequency w is an abbreviation of a line pair which is a frequency of a total of a white line and a black line.
The MTF may be more or less variable in a longitudinal direction of the rod lens array. The variation of the MTF makes an uneven electrostatic latent image (hereinafter referred to MTF unevenness) formed on the photosensitive drum even when rays having a uniform quantity are emitted from all the LEDs in the longitudinal direction or allows optical density unevenness to appear on an image and to be formed on a recording material in a worse case. Manufacturers of image forming apparatuses have made various contrivances to prevent the variation of MTF unevenness from appearing in images, thereby making it possible to provide uniform images which are not problematic in practical use.
Furthermore, the rod lens array is manufactured through processes which are described below:
1. Long rod lenses are manufactured in a large quantity.
2. The manufactured long rod lenses are arranged so as to form a sheet.
3. The sheet of the rod lenses is cut out in a length specified so as to obtain a desired TC length.
Owing to the manufacturing processes described above, rod lens arrays which are manufactured from an identical sheet can have MTF characteristics which are nearly equal to one another. Rod lens arrays which are cut out of portions vertically adjacent to each other in particular have a higher tendency to have the MTF characteristic equal to each other.
By composing an LED head of such an LED array and such a rod lens array and using the LED head as an exposing light source, it is possible to dispose the exposing light source in the vicinity of a photosensitive drum and compose an image forming apparatus more compact than a system which uses a semiconductor laser as an exposing light source.
In the case of an image forming apparatus which forms a full-color image generally consisting of four colors of cyan, magenta, yellow and black, MTF unevenness at an identical location in the longitudinal direction tends to be intensified by overlapping and finally appears as remarkable optical density unevenness even when the MTF unevenness appears as slight optical density which is scarcely remarkable on a monochromatic image.
When red is output from an exposing apparatus which produces optical density unevenness of 0.01 in yellow and magenta, for example, the unevenness is overlapped and appears as optical density unevenness on an image. The above-described unevenness in a skin color of a man""s face may look like a wrinkle which appears unattractive and does not truly express the intended image.
From a viewpoint of the manufacturing processes, on the other hand, it is pretty difficult to correct more strictly MTF unevenness of the rod lens array and the manufacturing cost inevitably increases when only satisfactory rod lens arrays are selected.
The present invention has been achieved in view of the above-described problems and has an object to provide an image forming apparatus and a manufacturing method of the image forming apparatus which are capable of suppressing optical density unevenness in an image.
Another object of the present invention is to provide an image forming apparatus and a manufacturing method of the image forming apparatus which are capable of making an optical density of image uniform in a direction perpendicular to a recording material conveying direction.
Still another object of the present invention is to provide an image forming apparatus comprising:
a plurality of light emitting heads, each of said heads having a rod lens array for focusing an image light on a photosensitive member,
wherein characteristics of modulation transfer function of each of the above-described rod lens arrays are different from each other.
Still another object of the present invention is to provide a manufacturing method of an image forming apparatus comprising steps of:
clustering a plurality of long rod lenses;
cutting a clustered long rod lens group into a plurality of short rod lens groups; and
disposing the short rod lens groups in different image forming apparatuses so that the plurality of short lens groups adjacent to each other in a direction of an optical axis are not disposed in an identical image forming apparatus.
Still another object of the present invention is to provide a manufacturing method of an image forming apparatus comprising steps of:
clustering a plurality of long rod lenses;
cutting a plurality of clustered long rod lens groups into a plurality of short rod lens groups respectively; and
disposing the short rod lens groups in different image forming apparatuses so that a plurality of short rod lens groups obtained from a long rod lens group are not disposed in an identical image forming apparatus.